1. Field of the Invention
The present invention relates to a mobile terminal having a roaming function.
2. Description of the Related Art
In the field of mobile communications, a service called roaming is implemented in order to receive a communication service of a non-subscription carrier outside the service area of a subscription carrier. For example, if a mobile terminal having such a roaming function cannot receive a radio signal from a subscription mobile communication network system, the mobile terminal searches a receivable mobile communication network system of another carrier. When the mobile communication network system which can be used as a roaming destination is found by this search, the mobile terminal proceeds to a standby state in synchronism with a base station in the found mobile communication network system. During roaming, the mobile terminal periodically performs system reselection control to try to change a connection destination to a subscription mobile communication network system (see, e.g., Jpn. Pat. Appln. KOKAI Publication No. 2000-13846).
However, if a conventional mobile terminal having such a roaming function tries to make an outgoing call or activate a browser in connection with a roaming destination mobile communication network system, this mobile terminal may actually be in the service area of a subscription carrier (HOME carrier). This is because the service areas of a subscription carrier and a roaming destination carrier overlap each other. The communication fee of the roaming destination carrier is generally higher than that of the subscription carrier. Hence, if a user unintentionally performs communication such as speech communication or Web access via the network of the roaming destination carrier, a problem may occur upon charging high communication fee later.
In order to solve such a problem, the mobile terminal may frequently perform system reselection control during roaming to return the connection destination to the subscription mobile communication network system as much as possible. However, with this operation, the mobile terminal consumes a large amount of power every system reselection control cycle, thereby shortening the battery life.